It has been previously reported from our laboratory that MPTP (1- methyl-4-phenyl-1,2,3,6-tetrahydropyridine) causes cell death in cultures of rat hepatocytes following its conversion to the toxic metabolite, MPP+ (1-methyl-4-phenyl-pyridinium). The reaction is catalyzed by MAO-B (monoamine oxidase-B) and deprenyl, a specific inhibitor of this enzyme, has been shown to prevent MPTP induced cell death. We have investigated the effect of structural modification of MPTP on the ability to induce cell death. N-acetyl 4'-amino MPTP was found to be virtually nontoxic in cultured hepatocytes, whereas N-butyl PTP, 4'-amino MPTP, and 2'-methyl MPTP were found to be toxic, but to a lesser degree than MPTP itself. The 4'-fluoro and 4'-chloro analogs evoked toxicities similar to that of the parent compound. Treatment of the hepatocytes with deprenyl was found to decrease the toxicities of N-butyl PTP, 4'- amino MPTP, and 4'-chloro MPTP, through the toxicities of the 2'- methyl analog remained largely unaffected. The conversion of all of these compounds, except for the N-acetyl amino analog into corresponding pyridinium metabolites by liver cells was confirmed by high pressure liquid chromatography and plasma desorption mass spectrometry. Deprenyl was found to inhibit their formation to varying degrees. Moreover, MPTP and its analogs served as substrates for monoamine oxidase in rat liver mitochondria to varying degrees. Even though deprenyl inhibited completely the conversion of MpTP to MPP+, the metabolism by MAO of most of the analogs were inhibited a lesser extent and to varying degrees by deprenyl. These findings indicate that even though the mechanisms of formation of pyridinium metabolites may differ, the formation of the pyridinium metabolites is necessary for the expression of toxicity.